This invention relates to ink jets, and more particularly, to ink jets of the demand type or impulse type.
Ink jets of the demand type include a transducer which is coupled to a chamber adapted to be supplied with ink. The chamber includes an orifice for ejecting droplets of ink when the transducer has been driven or pulsed by an appropriate drive voltage. The pulsing of the ink jet abruptly reduces the volume of the jet so as to advance the meniscus away from the chamber and form a droplet of ink from that meniscus which is ejected from the ink jet.
Demand ink jets typically operate by reducing or contracting the volume of the chambers in the rest state to a lesser volume in the active state when a droplet is fired. This contraction in the active state is followed by an expansion of the volume when the jet is returned to the rest state and the chamber is filled. Such a mode of operation may be described as a fire-before-fill mode.
FIG. 1 depicts chamber volume v as a function of time t in a demand ink jet operating in a fire-before-fill mode. Referring to FIG. 1, the time t.sub.0 represents the onset of the active state of the ink jet whereupon the volume of ink is reduced rapidly until time t.sub.1. This rapid reduction in volume produces the projection of a droplet on or about time t.sub.1. The contracted volume of the chamber continues with slight fluctuation until time t.sub.2 whereupon the contracted volume begins to expand until time t.sub.3. At time t.sub.3 marking the beginning of a rest state, the volume of the chamber is identical to that at time t.sub.0.
As shown in FIG. 1, the rest state continues for time d.sub.t ; between times t.sub.3 and t.sub.5 whereupon an active state is initiated resulting in the projection of another droplet. Operation at high droplet projection rates or frequencies will necessitate very short dead times d.sub.t corresponding to the inactive state. In other words, it may be necessary to initiate the active state so as to again contract the volume of the chamber at an earlier time t.sub.4 as depicted by dotted lines in FIG. 1. Generally speaking, higher droplet projection rates and/or frequencies are desirable but achieving such rates and/or frequencies with demand ink jets operating in a fire-before-fill mode as depicted by the waveform in FIG. 1 may create difficulties which will now be discussed with respect to FIGS. 2 through 4.
FIG. 2 depicts the meniscus position p as a function of time as the demand ink jet discussed with respect to FIG. 1 moves between the rest and active states. In this connection, it will be understood that the times t.sub.0 through t.sub.5 of FIG. 2 are conincident with the times t.sub.0 through t.sub.5 of FIG. 1 and the meniscus position p as depicted in FIG. 2 is a function of the chamber volume v as depicted in FIG. 1.
At time t.sub.0, the meniscus position p is at equilibrium corresponding with the position of the meniscus when the ink jet is in the rest state. As the ink jet moves into the active state and the chamber volume v contracts rapidly between times t.sub.0 and t.sub.1, the meniscus position moves forward resulting in the ultimate ejection of a droplet of ink at time t.sub.1. Immediately upon ejection of the droplet at time t.sub.1, the meniscus position p returns essentially to an equilibrium state as shown at time t.sub.2 while the volume v is still in the contracted state. At time t.sub.2, when the chamber volume v is expanding back to the volume of the ink jet in the rest state, the meniscus position retracts and is still in the retracted position at time t.sub.3 when the active state of the ink jet has terminated.
During the rest state corresponding to the dead time d.sub.t, the meniscus position advances back to the equilibrium position corresponding to the position of the meniscus in the rest state. As shown in FIG. 2, t.sub.5 has been chosen such that the meniscus position at time t.sub.5 has had an opportunity to return to the equilibrium position prior to the onset of the next active state and the ejection of another droplet of ink. However, if the next active state were to begin at time t.sub.4 resulting in the firing of a droplet of ink, the meniscus position would not yet have returned to the equilibrium state and the meniscus would abruptly advance at time t.sub.4 as shown in FIG. 2 with the result that the meniscus would reach a somewhat different position than the meniscus reached as a result of delaying the onset of the active state until time t.sub.5.
This variation in the position of the meniscus as a function of the duration of the dead time d.sub.t produces a variation in the droplet size and velocity which is undesirable in achieving the optimum in ink jet printing. The adverse effects with respect to droplet size may be readily appreciated with reference to FIGS. 3 and 4.
As shown in FIG. 3, a droplet of ink is fired when the meniscus is in an initial equilibrium position as shown in FIG. 3a. In particular, FIG. 3a shows a meniscus in the position depicted in FIG. 2 at time t.sub.5 FIGS. 3b through 3d show the advancement of the meniscus following time t.sub.5 including the formation of a droplet. FIG. 3e shows the ultimate droplet ejected.
If, however, the meniscus is at least partially retracted as at time t.sub.4 depicted in FIG. 4(a), a droplet of somewhat different size is formed as depicted by FIGS. 4b through 4e. More particularly, the formation of a droplet at the center of the meniscus in FIG. 4b results in a somewhat smaller droplet as depicted by FIG. 4e.
It will, therefore, be appreciated by reference to FIGs. 3 and 4 that droplets of different size may be generated utilizing a typical demand ink jet as a function of the dead time d.sub.t or duration of the rest state. Where high droplet projection rates or frequencies are desired, diminution of the dead time d.sub.t or duration of the active state will produce smaller droplets. On the other hand, larger droplets will be produced where the duration of the rest state or dead time d.sub.t is of some threshold duration.
FIG. 5 depicts a difference in velocity as a function of frequency which in turn is a function of the dead time d.sub.t. As shown, the droplet velocity increases from 0 kHz. up to 7 kHz. In other words, as the dead time dt is shortened so as to increase frequency, the droplet velocity varies as shown in FIG. 5.
There is an additional problem associated with the typical demand ink jet, i.e., a fire-before-fill jet. In many instances, such a jet will fire with the meniscus in the equilibrium state. Such a position is not particularly efficient from an operating standpoint since a greater volume contraction is necessary to generate a droplet of the same size and velocity because of the fluidic impedance of the droplet as compared with a droplet which is projected from a retracted meniscus wherein the fluidic impedance of the orifice is lessened.
Finally, the typical fire-before-fill demand ink jet suffers from an instability of the drop break off process. When the drop emerges from the orifice upon contraction of the chamber volume from an unretracted meniscus position which is necessary to avoid variations in droplet velocity and size, the droplet is more likely to attach to the edge of the orifice. This creates drop aiming problems which may be caused by geometric imperfections in the orifice edge. Firing from the equilibrium position of the meniscus is also more likely to result in ink spillover which will wet the face of the orifice as the droplet emerges also creating irregularities in droplet projection. Another disadvantage of such spillover is the probability of paper dust adhering to the jet face and causing a failure.